Two-stroke internal combustion engines are widely used in work apparatuses such as chain saws and brush cutters. This kind of portable power tools or work apparatuses, in general, have mounted an engine of a crankcase compression type that introduces air-fuel mixture into the crankcase and compresses it therein with a piston.
As already known, two-stroke internal combustion engines use air-fuel mixture to scavenge their combustion chambers. It is ideal that scavenging can be completed without any outflow of air-fuel mixture from the engine. Actually, however, there is the problem of “blow-by”, which is the phenomenon that air-fuel mixture partly flows out of the engine.
To cope with this blow-by problem, Japanese Patent Laid-open Publication JP 2001-355450 A (hereinafter referred to as Patent Document 1) proposes to introduce fuel-lean air-fuel mixture (hereinafter referred to as “lean air-fuel mixture” or “lean mixture” as well) into the combustion chamber at a final stage of the combustion stroke and next introduce fuel-rich air-fuel mixture (hereinafter referred to as “rich air-fuel mixture” or “rich mixture” as well) into the combustion chamber. More specifically, the engine has two mixture ports (a first mixture port and a second mixture port) opening into the cylinder bore, and these first and second mixture ports are opened and closed by the piston. The lean air-fuel mixture is introduced into the crankcase from the first mixture port and supplied to the combustion chamber through a scavenging passage that communicates with both the crankcase and the combustion chamber. On the other hand, the rich air-fuel mixture is introduced into the combustion chamber from the second mixture port.
Thus, this method scavenges the combustion chamber with lean mixture introduced there at the final stage of the combustion stroke. Therefore, air-fuel mixture exiting the engine is fuel-lean mixture containing less fuel, and the amount of fuel outflow can be reduced.
Besides the aforementioned scavenging that uses lean air-fuel mixture, further scavenging methods using air are known (Japanese Patent Laid-open Publications JP 2001-012249 A and JP 2001-239463 A, hereinafter referred to as Patent Document 2 and Patent Document 3, respectively). Patent document 2 discloses air-headed stratified scavenging. The air-headed stratified scavenging supplies air beforehand into a scavenging passage in communication with the combustion chamber and the crankcase. In each subsequent scavenging stroke, the air in the scavenging passage is first introduced into the combustion chamber. Thereafter, air-fuel mixture is introduced from the crankcase into the combustion chamber through the scavenging passage. Patent Document 3 discloses air-headed stratified scavenging as well.
More specifically, Patent Document 3 relates to a two-stroke internal combustion engine using a carburetor that has an air passage for air to pass through and a mixture passage for generating air-fuel mixture. The air passage and the mixture passage of the carburetor communicate with an air port and a mixture port, respectively, which both open into the cylinder bore. The air port and the mixture port are opened and closed by a piston, and air-fuel mixture is supplied to the crankcase through the mixture port. The piston has formed a groove in its cylindrical outer surface, which can communicate with the air port and is bifurcated into right and left branches along the outer circumference of the piston. Through this groove, the air port communicates with right and left scavenging passages, and the scavenging passages are charged with air. In a first portion of each scavenging stroke, the air in the scavenging passages is introduced into the combustion chamber. In a second portion of each scavenging stroke, the air-fuel mixture in the crankcase is introduced into the combustion chamber through the scavenging passages.
The scavenging methods as disclosed in Patent Document 2 and Patent Document 3 utilize the heading or leading air, but these methods have difficulties in deciding appropriate timing to start scavenging with the heading air and in regulating the amount of the heading air.
Patent document 2 suggests using a crankshaft web to open and close the inlet (an aperture facing to the crankcase) of the scavenging passage, thereby controlling a blowout timing for scavenging.
Furthermore, these scavenging methods using heading air are liable to suffer large changes in air-fuel ratio in the combustion chamber upon sudden acceleration, which often results in acceleration failure or engine stop. Taking it into consideration and to cope with sudden acceleration from idling, Patent Document 3 proposes to use a supplemental passage in an air valve provided in the air passage of the carburetor to ensure introduction of leading air into the combustion chamber even in the idling state.
The technique described in Patent Document 3 assumes that rich air-fuel mixture is needed for idling operation and lean air-fuel mixture with a relatively lean fuel ratio is used for normal operation. Under this assumption, this Patent Document 3 presumes that some of the leading air introduced into the combustion chamber will inevitably remain there even after scavenging, and proposes to introduce into the crankcase a richer air-fuel mixture during idling operation, which is richer in fuel ratio as much as compensating the shortage of fuel component in the remaining amount of the leading air. Therefore, when the engine is suddenly accelerated, since the richer air-fuel mixture introduced during idling operation partly remains in the crankcase, an air-fuel mixture containing the remaining part of the richer air-fuel mixture is introduced into the combustion chamber to ensure reliable acceleration of the engine.
Patent Document 2 and Patent Document 3 have the common idea of charging scavenging passages with heading air and using it for scavenging. In the process of charging the scavenging passages with the heading air, the leading portion of the air entering into the scavenging passages hits the air-fuel mixture having remained in the scavenging passages, and makes a portion of air containing fuel at its leading end. Therefore, even though “10” parts of the heading air, for example, are charged in each scavenging passage, its leading “2” parts, for example, inevitably contain fuel. As a result, in theory, even when ten parts of heading air are charged for scavenging, its two parts contain fuel component. This means that the fuel component contained in the two parts of the heading air is undesirably emitted by scavenging.